Formation and Occurrence of N-Chloro-2,2-dichloroacetamide, a

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The Formation and Occurrence of N-chloro-2,2dichloroacetamide, A Previously Overlooked Nitrogenous Disinfection Byproduct in Chlorinated Drinking Waters Yun Yu, and David A. Reckhow Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04218 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 25, 2016

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Environmental Science & Technology

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The Formation and Occurrence of N-chloro-2,2-dichloroacetamide, A

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Previously Overlooked Nitrogenous Disinfection Byproduct in Chlorinated

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Drinking Waters

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Authors: Yun Yu*1, David A. Reckhow2

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1. PhD, 18 Marston Hall, 130 Natural Resources Road, Department of Civil and

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Environmental Engineering, University of Massachusetts Amherst, Amherst MA, 01003-

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9293. E-mail: [email protected] Phone: 413-362-4918 (Corresponding Author).

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2. Professor, 18 Marston Hall, 130 Natural Resources Road, Department of Civil and

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Environmental Engineering, University of Massachusetts Amherst, Amherst MA 01003-

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9293. E-mail: [email protected]. Phone: 413-545-5392

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ACS Paragon Plus Environment


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ABSTRACT Haloacetamides (HAMs) are a class of newly identified nitrogenous disinfection byproducts

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(N-DBPs) whose occurrence in drinking waters has recently been reported in several DBP

15

surveys. As the most prominent HAM species, it is commonly acknowledged that 2,2-

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dichloroacetamide (DCAM) is mainly generated from dichloroacetonitrile (DCAN) hydrolysis

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because the concentrations of these two compounds are often well correlated. Instead of DCAM,

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a previously unreported N-DBP, N-chloro-2,2-dichloroacetamide (N-Cl-DCAM), was confirmed

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in this study as the actual DCAN degradation product in chlorinated drinking waters. It is

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suspected that N-Cl-DCAM has been erroneously identified as DCAM, because its nitrogen-

21

bound chlorine is readily reduced by most commonly-used quenching agents. This hypothesis is

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supported by kinetic studies that indicate almost instantaneous N-chlorination of DCAM even at

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low chlorine residuals. Therefore, it is unlikely that DCAM can persist as a long-lived DCAN

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decomposition product in systems using free chlorine as a residual disinfectant. Instead,

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chlorination of DCAM will lead to the formation of an equal amount of N-Cl-DCAM by forming

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a hydrogen bond between hypochlorite oxygen and amino hydrogen. Alternatively, N-Cl-DCAM

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can be produced directly from DCAN chlorination via nucleophilic addition of hypochlorite on

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the nitrile carbon. Due to its relatively low pKa value, N-Cl-DCAM tends to deprotonate under

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typical drinking water pH conditions and the anionic form of N-Cl-DCAM was found to be very

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stable in the absence of chlorine. N-Cl-DCAM can, however, undergo acid-catalyzed

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decomposition to form the corresponding dichloroacetic acid (DCAA) when chlorine is present,

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although those acidic conditions that favor N-Cl-DCAM degradation are generally atypical for

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finished drinking waters. For these reasons, N-Cl-DCAM is predicted to have very long half-

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lives in most distribution systems that use free chlorine. Furthermore, an analytical method using

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ultra performance liquid chromatography (UPLC)/ negative electrospray ionization (ESI-)/

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quadrupole time-of-flight mass spectrometry (qTOF) was developed for the detection of a family

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of seven N-chloro-haloacetamides (N-Cl-HAMs). Combined with solid phase extraction (SPE),

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the occurrence of N-Cl-DCAM and its two brominated analogues (i.e., N-chloro-2,2-

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bromochloroacetamide and N-chloro-2,2-dibromoacetamide) was quantitatively determined for

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the first time in 11 real tap water samples. The discovery of N-Cl-DCAM or more broadly

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speaking, the N-Cl-HAMs in chlorinated drinking waters is of significance because they are

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organic chloramines, a family of compounds that is perceived to be more toxicologically potent

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than halonitriles (e.g., DCAN) and haloamides (e.g., DCAM), and therefore they may pose

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greater risks to drinking water consumers given their widespread occurrence and high stability.

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INTRODUCTION

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Disinfection of drinking water provides an important barrier in the control of pathogenic

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microorganisms and therefore is an effective means of protecting consumers against waterborne

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diseases. However, the presence of a disinfectant residual can lead to the formation of unwanted

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carcinogenic disinfection byproducts (DBPs). To date, approximately 600-700 DBPs have been

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identified in drinking waters from the use of major disinfectants (i.e., chlorine, chloramines,

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chlorine dioxide, etc.) as well as their combinations.1-5 However, none of those previously

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reported DBPs has been recognized to have sufficient carcinogenic potency to account for the

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cancer risks that are projected from epidemiological studies.6 Meanwhile, haloacetamides

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(HAMs) have received a lot of attention as an emerging group of nitrogenous DBPs (N-DBPs)

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mainly because they are an order of magnitude more genotoxic and two orders of magnitude

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more cytotoxic than the corresponding haloacetic acids (HAAs),7 which are currently regulated

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by the USEPA as surrogates for drinking water toxicity.

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The occurrence of HAMs was first reported in a 2000-2002 DBP survey that was conducted

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at 12 US drinking water treatment plants.4,8 The median and maximum concentrations for a

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group of five chlorinated and brominated HAMs were 1.4 µg/L and 7.4 µg/L, respectively,

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among which 2,2-dichloroacetamide (DCAM) occurred at the highest levels with a median

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concentration of 1.3 µg/L.4,8 More recently, 2,2,2-trichloroacetamide (TCAM) was found to be

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present at a lower level than DCAM in samples collected from 20 English drinking water supply

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systems (respective median concentrations for these two HAMs were 0.4 µg/L and 0.6 µg/L).9

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Furthermore, the concentrations of both DCAM and TCAM were noted to be slightly higher in

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distribution systems than in finished waters,9 even though the observed differences were too

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small to make any significant inferences about their stability during drinking water distribution. 4


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Perhaps most importantly, in all those surveys, HAMs exhibited strong positive correlations

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with the corresponding haloacetonitriles (HANs) and these two groups of compounds were often

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detected at comparable levels.4,9,10 This is consistent with the prevailing understanding that

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HAMs in drinking waters result predominantly from base-catalyzed HAN hydrolysis.11,12 For

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instance, laboratory research has verified that dichloroacetonitrile (DCAN) can hydrolyze to

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DCAM and ultimately to dichloroacetic acid (DCAA) in the absence of free chlorine when pH is

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above neutral.13 However, in the presence of chlorine, hypochlorite (i.e., OCl-) has been

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recognized as the dominant contributor to DCAN decomposition and the reaction between

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DCAN and free chlorine forms N-chloro-2,2-dichloroacetamide (N-Cl-DCAM) as the putative

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reaction product.13 In fact, the formation of this halogenated nitrogenous compound had been

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proposed earlier in the 1990s as one of the major DCAN degradation products.14 Nevertheless,

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the formation of N-Cl-DCAM has not been substantiated and its presence in drinking waters has

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never been recognized or reported. Furthermore, the formation of an N-Cl-DCAM analogue, N,2-

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dichloroacetamide (or N-chloro-2-monochloroacetamide) has recently been observed from the

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reaction between chloroacetaldehyde and monochloramine.15 This N-chloro-haloacetamide (N-

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Cl-HAM) was found to be very unstable, undergoing rapid dechlorination to the corresponding

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2-chloroacetamide (or monochloroacetamide) in the presence of sodium thiosulfate.15 This

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finding raises the following question: will the other N-Cl-HAM species, especially N-Cl-DCAM,

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exhibit similar behavior when a reducing agent is added so that they will be chemically reduced

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to the corresponding HAMs during sample preservation? Moreover, if N-Cl-HAMs can initially

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form but subsequently convert into HAMs, then what proportion of the latter that has previously

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been identified and reported was actually due to N-Cl-HAM dechlorination? Therefore, the

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extent to which N-Cl-HAMs will be reduced by the addition of common quenching agents, such

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as sodium sulfite, ascorbic acid, ammonium chloride, and sodium thiosulfate needs to be

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clarified.

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For these reasons, the major objectives of this study were to confirm the existence of N-Cl-

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DCAM in chlorinated drinking waters, and to quantitatively characterize and reconcile its

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formation kinetics with the degradation kinetics for both DCAN and DCAM. Furthermore, with

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its third chlorine bound to the amide nitrogen, N-Cl-DCAM can be defined as an organic

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monochloramine. Since organic chloramines are capable of transferring Cl(+I) to other amino

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groups such as those making up the exocyclic nitrogens in DNA and RNA,16 it is commonly

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acknowledged that they are toxicologically active. Thus, they may pose special health concerns

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to consumers regarding chronic diseases6 if they have sufficient stability to persist through

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drinking water distribution. For this reason, the stability of N-Cl-DCAM was evaluated under

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typical drinking water pH conditions both with and without the presence of chlorine.

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Additionally, the impact of commonly-used reducing agents on the integrity of N-Cl-DCAM was

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also assessed in this study. Finally, in order to demonstrate the existence of the N-Cl-HAMs, it

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was necessary to develop an analytical method for their quantification at microgram per liter

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levels in finished drinking waters. Once this was done, a set of tap water samples collected from

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seven private residences in the US were analyzed to examine the presence of N-Cl-HAMs in

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actual drinking water supplies.

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MATERIALS AND METHODS

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Chemicals and Reagents

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2,2-dichloroacetamide (DCAM) and 2,2,2-trichloroacetamide (TCAM) were purchased from

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Sigma-Aldrich (St. Louis, MO). Bromochloroacetamide (BCAM), dibromoacetamide (DBAM),

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bromodichloroacetamide (BDCAM), dibromochloroacetamide (DBCAM), and

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tribromoacetamide (TBAM) were supplied by CanSyn Chem. Corp. from Canada. General

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laboratory chemicals including Optima LC/MS grade organic solvents and formic acid (FA)

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were obtained from Fisher Scientific (Pittsburgh, PA). Purified N-chloro-haloacetamide (N-Cl-

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HAM) standard compounds are not commercially available, and therefore they were individually

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prepared by reacting an equal stoichiometric amount of free chlorine with the corresponding

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haloacetamides (i.e., Cl2/N=1:1) ,15,17 with the pH of both solutions adjusted to 9.0 before mixing.

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Chlorination of Dichloroacetamide

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All DCAM reaction solutions were prepared in ultrapure Milli-Q water (EMD Millipore

123

Corp.) containing 10 mM phosphate buffer and were adjusted to the desired pHs with sodium

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hydroxide or hydrochloric acid. At the beginning of each chlorination experiment, 3 mL of

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DCAM reaction solution (0.505 mM) was introduced into a quartz cuvette with a 1 cm path

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length. Chlorination of DCAM was conducted by adding a small volume (30 µL) of acidified

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sodium hypochlorite solution (50.5 mM as Cl2) into the aforementioned DCAM reaction solution

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(3 mL in a cuvette), so that the initial concentration for both reactants was 0.5 mM. Acidified

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sodium hypochlorite solutions (50.5 mM as Cl2) were prepared on the day of use by diluting the

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sodium hypochlorite stock solution (5.65%-6%, laboratory grade, Fisher Scientific), followed by

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hydrochloric acid neutralization to the predetermined pHs, prior to which, the actual free

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chlorine concentration in the stock solution was standardized based on the N,N-diethyl-p-

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phenylene diamine (DPD)-ferrous ammonium sulfate (FAS) titrimetric method (EPA Method

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330.4). Immediately after the introduction of chlorine, the cuvette was capped and quickly

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inverted for three times to ensure even distribution of the reactants before being placed in the

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spectrophotometer. Absorption spectrum was scanned once every 5 seconds in the continuous 7



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kinetic mode from 200 nm to 400 nm using an Agilent 8453 diode array UV-visible

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spectrophotometer. All DCAM chlorination reactions were monitored at ambient room

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temperature (i.e., 20 °C). Reaction rate constants were determined from the kinetic UV

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absorbance measurements at 292 nm.

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Stability of N-chloro-2,2-dichloroacetamide

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The stability of N-Cl-DCAM was assessed in phosphate buffered solutions (10 mM, pH 4-8)

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with and without the presence of chlorine. Initial N-Cl-DCAM concentration was 40 µM and a

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small volume of acidified sodium hypochlorite solution was introduced at the beginning of each

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test so that the initial total chlorine concentration was also 40 µM. The chlorinated and

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unchlorinated N-Cl-DCAM solutions were repeatedly injected into the ultra performance liquid

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chromatography (UPLC)/ negative electrospray ionization (ESI-)/ quadrupole time-of-flight mass

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spectrometer (qTOF) once every 15 minutes for a total of 8 hours. Reduction of N-Cl-DCAM by

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sodium sulfite, sodium thiosulfate, ammonium chloride, and ascorbic acid was investigated in

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the same fashion via repeated sample injections into the UPLC/ESI/qTOF.

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Sample Pretreatment

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For the quantification of a group of seven N-Cl-HAMs, a solid phase extraction (SPE)-ultra

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performance liquid chromatography/mass spectrometry (UPLC/MS) method was developed

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during the course of this study. Before analysis, N-Cl-HAMs were first concentrated through

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SPE using the Oasis mixed-mode, reversed-phase, strong anion-exchange (MAX) cartridges (60

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mg, 3 mL, 30 µm; Waters, Milford, MA) that were mounted on an Agilent VacElut SPS 24 SPE

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manifold. Prior to sample loading, each MAX cartridge was conditioned with 3 mL of methanol

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followed by one wash using 3 mL of ultrapure Milli-Q water. Each sample (100 mL) was drawn

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through the cartridge under vacuum at a flow rate of approximately 1 mL/min. After sample

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loading, the cartridges were washed with 2 mL of methanol/NH4OH (v/v=95/5) and then dried

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for 1 minute under vacuum. Subsequently, the retained N-Cl-HAMs were eluted with 2 mL of

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acetonitrile/water (v/v=90/10, with 25% formic acid). The acetonitrile extract was reconstituted

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by adding 0.5 mL of water/NH4OH (v/v=85/15) and was then evaporated down to 1.0 mL under

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a gentle nitrogen stream (TurboVap LV).

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Ultra Performance Liquid Chromatography/Quadrupole Time-of-Flight Mass Spectrometry

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An ACQUITY UPLC (Waters, Milford, MA) system was used for LC separation with an

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ACQUITY UPLC HSS T3 column (1.8 µm, 100 Å, 2.1×100 mm; Waters), coupled with a 1.8

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µm, 2.1×5 mm VanGuard pre-column (ACQUITY UPLC HSS T3; Waters). Column

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temperature was maintained isothermally at 35 °C. The mobile phases were 5 mM ammonium

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acetate (solvent A) and 100% methanol (solvent B) at a constant flow rate of 0.3 mL/min. The

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initial gradient was 0-2 min, 5% B, curve 6; increased from 5% to 90% B between 2 and 7 min,

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curve 6; 7-8 min 90% B, curve 6; switched back to 5% B in 0.1 min, curve 11; 11-15 min for

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equilibration, 5% B. The injection volume for each sample was 5 µL. A quadrupole time-of-

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flight mass spectrometer (Xevo G2-XS qTOF; Waters) with an electrospray ionization (ESI)

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source was used to obtain the exact masses of N-Cl-HAM parent ions. Negative ESI-TOFMS

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mode was applied with typical conditions optimized as follows: capillary voltage 2.50 kV;

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sampling cone, 25 arbitrary units; source offset, 80 arbitrary units; source temperature, 120 °C;

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desolvation temperature 400 °C; cone gas, 80 L/hour; desolvation gas flow, 800 L/hour.

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Method Validation

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To determine the method detection limits (MDLs) and recoveries for the seven N-Cl-HAM

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analytes using the SPE-UPLC/ESI/qTOF method, three sets of tap water samples (100 mL each)

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were prepared: (1) eight calibration standards spiked with seven N-Cl-HAMs; (2) seven replicate

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samples spiked with 0.02 µM of each N-Cl-HAM; (3) unspiked blanks. All standards and

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samples were extracted and analyzed at the same time using the method described above. The

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SPE recovery rate for each N-Cl-HAM was determined according to the standard addition

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method.18 Furthermore, to validate the SPE-UPLC/ESI/qTOF method, 11 tap water samples

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collected from seven private residences in the US were analyzed for the quantification of N-Cl-

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HAMs. Prior to sampling, 100 mg of ammonium chloride (NH4Cl) was added to each 1 L glass

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bottle as preservative. All samples were collected without headspace, stored at 4 °C, extracted

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within 72 hours, and analyzed by UPLC/ESI/qTOF immediately after sample pretreatment (i.e.,

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SPE).

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RESULTS AND DISCUSSION

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Identification and Verification of N-chloro-2,2-dichloroacetamide and N-chloro-

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haloacetamides

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The discovery of N-Cl-DCAM stemmed from a preliminary kinetic study where the stability

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of DCAM was evaluated under a range of pH conditions with and without the presence of

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chlorine. When DCAM was chlorinated and residual chlorine was quenched at prescribed

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reaction times by ascorbic acid, no significant decrease in DCAM concentration was observed.

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In contrast, residual DCAM was undetectable at identical reaction times when chlorinated

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samples were immediately analyzed by liquid-liquid extraction-gas chromatography/mass

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spectrometry (LLE-GC/MS; unpublished method) without the addition of any reducing agent.

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This suggested that DCAM chlorination might have formed a labile reaction intermediate, which

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was converted back into the initial DCAM as a result of ascorbic acid addition. In order to

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identify this reaction intermediate, unquenched DCAM chlorination solution was directly infused

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into the high-resolution quadrupole time-of-flight mass spectrometer (Xevo G2-XS qTOF).

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Under negative electrospray ionization (ESI-), a unique isotope cluster was observed, reflecting

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the presence of three chlorine atoms in this unknown compound. In fact, the mass spectrum of

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the unknown was found to agree with that of 2,2,2-trichloroacetamide (TCAM) in both exact

209

masses and their isotopic patterns.

210

Nonetheless, TCAM behaved very differently in many ways from this unidentified

211

compound and therefore is unlikely the aforementioned labile reaction intermediate formed

212

during DCAM chlorination. First of all, the unknown compound was well retained on a UPLC

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column (Waters ACQUITY HSS T3) with a stationary phase that promotes polar compound

214

retention. On the contrary, TCAM standard compound was eluted near the dead volume over the

215

entire mobile phase composition range, which indicates different chemical polarities between

216

these two. More importantly, TCAM didn’t dechlorinate to form DCAM in the presence of

217

ascorbic acid, whereas conversion of the unknown to DCAM was found to be a more generic

218

result from the addition of not only ascorbic acid but also other reductants as well (e.g.,

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potassium iodide, sodium sulfite, and sodium thiosulfate). Lastly, this DCAM chlorination

220

product exhibited a very similar behavior as inorganic dichloramine (i.e., NHCl2), both of which

221

slowly oxidized iodide to triiodide that further reacted with N,N-Diethyl-p-phenylene diamine

222

(DPD) to produce a relatively stable free radical species with an intense pink color. In contrast,

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TCAM didn’t react with DPD either directly or indirectly via the triiodide intermediate to form

224

the colored free radicals.

11



225

Alternative to TCAM formation via chlorine substitution on the alkyl carbon, chlorination of

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DCAM may also result in bonding of chlorine to the amide nitrogen,19,20 forming N-chloro-2,2-

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dichloroacetamide (N-Cl-DCAM) as a constitutional isomer of TCAM (Scheme 1). Particularly,

228

trihaloacetamides (THAMs) including trichloroacetamide (TCAM), bromodichloroacetamide

229

(BDCAM), dibromochloroacetamide (DBCAM), and tribromoacetamide (TBAM) cannot be C-

230

chlorinated due to the absence of a substitutable hydrogen on the trihalogenated tertiary carbon.

231

However, N-chlorination of THAMs may otherwise be possible, leading to the formation of N-

232

chloro-trihaloacemides (N-Cl-THAMs) that will be distinctively tetrahalogenated. In this regard,

233

if THAMs can be further chlorinated to form those hypothesized tetrahaloacetamides, the HAM

234

N-chlorination pathway can therefore be verified and N-Cl-DCAM can be confirmed as the

235

chlorination product for DCAM. To substantiate this speculation, seven dihalogenated and

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trihalogenated HAMs (i.e., DCAM, BCAM, DBAM, TCAM, BDCAM, DBCAM, and TBAM)

237

were chlorinated and the resulting HAM chlorination solutions were individually infused into the

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qTOF mass spectrometer to screen for N-Cl-HAMs.

239

240 241

242

Scheme 1. 2,2,2-trichloroacetamide (TCAM) and its constitutional isomer, N-chloro-2,2dichloroacetamide (N-Cl-DCAM). Figure 1 shows the obtained isotope clusters for the four THAM chlorination products on the

243

bottom row. All four isotopic distributions indicate the presence of a combination of four

244

halogen atoms (i.e., chlorine or bromine) in their molecular structures. Furthermore, the 12


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245

measured masses for all [M-H]- ions were in perfect agreement with the calculated values for N-

246

Cl-THAMs. Therefore, it can be concluded that THAMs can be further N-chlorinated by free

247

chlorine on the amide nitrogen. Following the same mechanism, chlorination of

248

dihaloacetamides (DHAMs), including DCAM, BCAM, and DBAM, will produce the

249

corresponding N-chloro-dihaloacetamides (N-Cl-DHAMs) instead of their THAM isomers (i.e.,

250

TCAM, BDCAM, and DBCAM). As a result, the formation of N-Cl-DCAM from DCAM

251

chlorination can thus be confirmed.

252

253

Figure 1. Obtained isotope clusters for the seven HAM chlorination products (i.e., N-Cl-

254

HAMs) using Xevo G2-XS qTOF. All N-Cl-HAMs were formed individually by reacting an

255

equal stoichiometric amount of free chlorine with the corresponding HAM (i.e., 100 µM Cl2:100

256

µM HAM). For each N-Cl-HAM, the masses measured (shown in black) were compared with the

257

values calculated (shown in red) and all halogen isotopes are indicated by the blue arrows. 13



258 259

Dichloroacetamide Chlorination Kinetics In the preliminary study, it was noted that residual chlorine was exhausted almost

260

instantaneously when DCAM was chlorinated by an equal stoichiometric amount of chlorine,

261

implying that the formation of N-Cl-DCAM from DCAM chlorination might be very rapid. To

262

quantitatively determine the rate at which N-Cl-DCAM is formed, DCAM N-chlorination

263

kinetics was investigated spectrophotometrically by reacting DCAM with same molar

264

concentration of aqueous chlorine (i.e., [DCAM]0=[Cl2]0=0.5 mM) at four different pHs (i.e., pH

265

6, 7, 8, and 9). UV absorbance at 292 nm was monitored over reaction time at a sampling

266

frequency of once every five seconds. At each reaction time point, the concentrations of residual

267

hypochlorous acid (i.e., HOCl), hypochlorite (i.e., OCl-), DCAM, and formed N-Cl-DCAM can

268

be determined as follows based on their individual molar absorptivities at the given wavelength

269

(i.e., ): , = , [] + , [ ] + , [] + , [] (1)

270

Compared to chlorine, neither DCAM nor N-Cl-DCAM caused significant UV absorption at

271

292 nm when their concentrations were controlled the same (Figure S1). This is indicative of

272

very low molar absorptivities of these two compounds at this specific wavelength (i.e., 292 nm),

273

even though the actual values were not determined in this study. As a result, total residual

274

chlorine concentration at each reaction time (i.e., ) can be calculated using Eq. 2, assuming

275

negligible contributions from both residual DCAM and formed N-Cl-DCAM to the total

276

absorbance (i.e., , ) at 292 nm (i.e., ,!"! [] ≈ ,!"! [] ≈ 0 in

277

Eq.1).

14


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=

%&, =

278

, (2) %&, ,!"! + %', ,!"!

[ ) ] *+, ; %', = (3) ) *+, + [ ] *+, + [ ) ]

In Eq. 2, the respective molar absorptivities of hypochlorite and hypochlorous acid at 292 nm

279

are 350.2 M-1cm-1 and 26.95 M-1cm-1.21,22 The two alpha values (denoted as %& and %' in the

280

following discussion for simplicity) represent the fractions of total residual chlorine that are in

281

the form of hypochlorous acid (i.e., %& ) and hypochlorite (i.e., %' ), respectively. At any given pH,

282

those two fractions can be determined using Eq. 3 and a dissociation constant *+, for

283

hypochlorous acid of 10-7.582 at 20°C (Morris, 1966).

284

As is shown in Figure 2(a), residual chlorine concentrations were consistent with a rate law

285

that is second-order in total chlorine (i.e., ). Furthermore, DCAM exhibited a fixed demand of

286

1 mM Cl2 per 1 mM DCAM in the presence of excess molar equivalents of chlorine (Figure

287

2(b)), suggesting a 1:1 reaction stoichiometry between DCAM and total free chlorine. Perhaps

288

most importantly, it is clear from Figure 2(a) that hypochlorite is the only reactive form of

289

chlorine in DCAM N-chlorination, because chlorine decay was nearly undetectable at pH 6 but

290

was substantially accelerated when pH was above .*+, of 7.582.23 The specific participation

291

of hypochlorite in DCAM chlorination is consistent with the amide N-chlorination mechanism,24

292

which indicates that formation of a hydrogen bond by the amino hydrogen with the hypochlorite

293

oxygen is the rate-limiting step for this type of reactions. Moreover, hypochlorite is probably the

294

only chlorinating agent since the oxygen atom in hypochlorous acid does not have enough

295

electron-donating tendency to form such a hydrogen bond with the amino hydrogen.24 Therefore,

15



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the full second-order DCAM chlorination kinetics can be described as follows, which reflects the

297

particular involvement of hypochlorite in this reaction: /0

+ 1223 (4) 5[] 5 = = 789 [][ ] = 789 ∙ %' = 789 %' ∙ ! 56 56 ; ?

Formation and Occurrence of N-Chloro-2,2-dichloroacetamide, a

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